One often-misunderstood idea in evolutionary theory is survival of the fittest. Most think this refers to a single "best" option being selected. Often an alternate process -- dubbed balancing selection -- occurs in which several variants, each with unique advantages and disadvantages, are preserved.

The textbook example of balancing selection is sickle cell anemia. People born with the special hemoglobin variant suffer a variety of symptoms including poorer endurance, but the mutation grants immunity to malaria, a nasty disease that infects more than 200 million people worldwide a year. For that reason both sickle cell and healthy hemoglobin genes have been preserved to "keep mankind's options open", so to speak.

Professor Przeworski describes, "When we looked for genetic clues pointing to other, more ancient, examples of balancing selection, we found strong evidence for at least six such regions and weaker evidence for another 119 -- many more than we expected. We don't yet know what their functions are. [Clues point to their use in pathogen defense] but which pathogens, what immune processes, we don't know."

The six regions do not code for protein sequences. Protein coding sequences make up only a small part of the overall genome, which is largely composed of regulatory and preservationist sequences.

The team looked at the genomes of 59 humans from sub-Saharan Africa and 10 chimpanzees from Western Africa. The results give clues about how evolution helped both chimps and humans keep up with the fight against pathogens, a natural "arms race" in which both species often face similar classes of enemies.

Study first author Ellen Leffler, a graduate researcher says the fact that the set of "options" are preserved in both the humans and chimpanzees shows that they play an important role and are not purely random. She comments, "[The genes] must have been functionally important over evolutionary time."

The researchers used special codes to sort clusters of gene varieties and map how the human gene variety compared to that of their primate relatives.

Researchers say the finding of similar clusters of genes is very important as such preserved genetic variety is "extremely rare". One example of such a mechanism examined in previous studies is the major histocompatibility complex (MHC), a critical part of the immune system that helps distinguish between different kinds of pathogens. In a 2012 study Professor Przewski's team showed that humans and gibbons share the same ABO-blood type varieties.